The present invention relates to alumina based ceramic cutting tools containing zirconia and silicon carbide. It especially relates to ceramic cutting tools useful in the high speed, rough machining of soft steels and ductile or malleable cast irons.
In the past, zirconia has been added in specified amounts to selected alumina-silicon carbide whisker reinforced compositions described in U.S. Pat. Nos. 4,534,345 and 4,507,224 to provide enhanced fracture toughness and/or flexural strength (see: U.S. Pat. Nos. 4,657,877 and 4,749,667; Japanese Patent Publication No. Sho 62-265182; Clausen et al, "Whisker-Reinforced Oxide Ceramics," Journal de Physique Colloque Cl, Supplement au No. 2, Tome 47, February 1986, Pages Cl-693 to Cl-702; Becher et al, "Toughening of Ceramics by Whisker Reinforcement," Fracture Mechanics of Ceramics 7, ed. by Bradt et al, Plenum Press, New York (1986), Pages 61-73).
It has been indicated that the zirconia should be in the monoclinic and/or (metastable) tetragonal phase to obtain improved fracture toughness and/or flexural strength. It has been further indicated that the metastable tetragonal phase is obtained by reducing zirconia particle size or through the use of a cubic zirconia stabilization promoter such as yttria, calcia, magnesia and the rare earth oxides in amounts below that required to fully stabilize the cubic zirconia phase.
Cutting tools composed of a variety of compositions containing alumina, zirconia and silicon carbide whiskers with or without other additives have been proposed (see: European Patent Application No. 86107916.8 (published Jan. 21, 1987 as No. 0208910); U.S. Pat. No. 4,749,667; "Multitoughening Ceramic," Techno Japan, Vol. 19, No. 10, October 1986, Page 78; and European Patent Application No. 86301597.0 published Sept. 17, 1986, as No. 0194811).
Where an indication is given as to the material being cut by these tools, these tools have been applied to the machining of cast irons, hardened steels and nickel based superalloys. These are all materials that have relatively low reactivity with the silicon carbide in the cutting insert at the high temperatures encountered during metalcutting. None of the foregoing documents teaches or suggests that, for metalcutting inserts for use in the high speed roughing of soft steels, cutting performance can be significantly improved by controlling the alumina based ceramic composition to within the combination of critical ranges now discovered by the applicants for zirconia, magnesia, silicon carbide and tetragonal zirconia contents.